1. Field of the Invention
The invention relates to a process for preparing N,N'-disubstituted p-quinonediimines of general formula I ##STR3## where R.sup.1 and R.sup.2 are identical or different and are each a phenyl group and/or a straight-chain and/or branched alkyl group having from 3 to 8 carbon atoms and/or a cyclohexyl group, and also their use. The invention further relates to organosilanes containing methacryloxy or acryloxy groups and having general formula II ##STR4## where R.sup.3 is a hydrogen atom or a methyl group, R.sup.4 and R.sup.5 are identical or different alkyl groups having from 1 to 4 carbon atoms, m is equal to 0, 1 or 2, a process for their preparation and a process for their stabilization. The organosilanes of general formula II are hereinafter also referred to as acrylsilanes.
2. Discussion of the Background
N,N'-disubstituted p-quinonediimines of general formula I are not commercial products, but nevertheless are compounds having interesting chemical and physical properties.
In the known processes for preparing N,N'-disubstituted p-quinonediimines, hereinafter also referred to as simply diimines, the starting materials used are the corresponding N,N'-disubstituted p-phenylenediamines which are converted into the corresponding diimines by the action of oxidizing agents. Hereinafter, the N,N'-disubstituted p-phenylenediamines are also referred to simply as diamines. The processes for preparing the diimines gives, according to the prior art, either only low yields or impure products, are uneconomical, or extremely toxic compounds are used.
Thus, the preparation of N,N'-diphenyl-p-quinonediimine from the corresponding diamine by oxidation with chromic acid in dilute acetic acid solution has been described (Chem. Ber. 46 (1913) p. 1853). The diimine formed is obtained in an amorphous form which is difficult to purify and has to be purified by repeated recrystallization. As byproducts, there are formed relatively large amounts of a chromium-containing, aqueous, acetic acid solution which can be disposed of only with difficulty.
It is further known that N,N'-diphenyl-p-quinonediimine can be prepared by shaking a benzene solution of N,N'-diphenyl-p-phenylenediamine with an aqueous potassium ferricyanide solution (P. Feichtmayr and F. Wurstlin, Berichte der Bunsengesellschaft Vol. 67 (1963) p. 435). Since the reactants are present in separate phases, the reaction proceeds very slowly and the yields are unsatisfactory.
The same process is also described for the preparation of N-phenyl-N'-isopropyl-p-quinonediimine and N-phenyl-N'cyclohexyl-p-quinonediimine (L. Kotulak et al., Collect. Czech. Chem. Commun. 48 (1983) 12, p. 3384-3395).
The use of silver oxide in place of potassium ferricyanide as an oxidizing agent has also been proposed (L. Kotulak et al., Collect. Czech. Chem. Commun. 48 (1983) 12, p. 3384-3395). However, silver oxide is a very expensive and difficult-to-handle chemical.
The preparation of N,N'-disubstituted p-quinonediimines by air oxidation of the corresponding diamines is also known. Thus, N,N'-diphenyl-p-phenylenediamine is converted into a diimine by atmospheric oxygen in the presence of high excesses of azobisisobutyronitrile (AIBN) (C. E. Boozer et al., Journ. Amer. Chem. Soc., 77 (1955) p. 3233). Disadvantages of this process are the toxicity of the AIBN used, the use of chlorobenzene as solvent and the low yields.
U.S. Pat. No. 2,118,826 describes the oxidation of N,N'-disubstituted p-arylenediamines to the corresponding p-quinonediimines using air in the presence of solid alkali metal or alkali earth metal oxides, hydroxides, carbonates or amides. Disadvantages are the solid-state reaction, the high reaction temperatures of from 130 to 180.degree. C. and also long reaction times of up to 8 hours, with considerable decomposition of the diimines formed taking place.
Although the additional use of heavy metal salts as catalysts can lower the reaction temperatures, these salts can be removed only with difficulty from the N,N'-disubstituted p-quinonediimines obtained.
EP-B 0 437 653 teaches the synthesis of organosilanes of general formula II by reacting alkali metal salts of methacrylic acid or acrylic acid with chloropropylsilanes of general formula III, ##STR5## where R.sup.4, R.sup.5 and m are as defined above, in the presence of phase-transfer catalysts of the tetraalkylammonium type, with N,N'-diphenyl-p-phenylenediamine being used as stabilizer in the examples presented to prevent undesired polymer formation, in particular the formation of "popcorn" polymer. It is known that, for example, heavy metal salts can trigger the undesired polymerization of the acrylsilanes and also can accelerate the siloxane formation of alkoxysilane functions. The purification of the acrylsilanes is carried out by distillation.
The synthesis of the acrylsilanes by the phase-transfer catalysis process, hereinafter also referred to as PTC process, generally comprises four individual steps:
Stage 1: Preparation of the alkali metal methacrylate or acrylate PA0 Stage 2: Reaction of these salts with chloropropylalkoxysilanes in the presence of the phase-transfer catalyst to give the acrylsilane PA0 Stage 3: Separation of the accompanying product alkali metal chloride PA0 Stage 4: Distillative workup of the crude product
The addition of the stabilizer system is generally carried out after the first process stage, the preparation of the alkali metal acrylates.
Not very advantageous in the PTC process is the fact that the above-mentioned stabilizer does not go into the vapor phase in the fourth stage, the distillative workup of the crude organosilanes of general formula II, owing to its low volatility. This can lead, in particular for continuous process operation, to the formation of "popcorn" polymer in the distillation columns or the pipe systems, resulting in operating faults or even to damage of the plant.
DE-C 38 32 621 discloses, for improving the stabilization of acrylsilanes prepared by the process of addition of trialkylsilanes to allyl methacrylate using H.sub.2 PtCl.sub.6 as catalyst, stabilizer combinations of N,N'-disubstituted p-phenylenediamines and sterically hindered phenols, with, in the distillate workup of the crude acrylsilanes, the diamines taking over the stabilization of the liquid phase and the volatile phenols taking over the stabilization of the vapor phase. The industrial implementation of this process provides for the addition of the stabilizers prior to or after completion of the synthesis steps. However, the achievable yields of acrylsilanes are unsatisfactory.
In addition, the stabilizer combination of N,N'-disubstituted diamines and sterically hindered phenols proves to ineffective when the acrylsilane synthesis is carried out by the PTC process. Here, addition of the stabilizer combination in the course of or after completion of the first process stage, the preparation of the alkali metal methacrylate or acrylate from alcoholic alkali metal alkoxide solution and methacrylic acid or acrylic acid customary in the process, results in the phenolic component being converted into non-volatile alkali metal phenoxide, indeed independently of whether the acid or the alkoxide is initially charged in the preparation of the salt. In the distillative workup of the crude acrylsilanes, the free, volatile phenol is thus not available, causing "popcorn" polymer formation in the columns and giving a poorer product yield.